How to unlink a node group datablock?
I'm pretty sure that no item has a link to other items that it is used in, you need to loop through arrays and see if the item you are looking for exists.

If you only have a couple of materials in your blend file then you may want to press Home to bring every node into view - it may be located way off to the side. It could also be hiding under another node or inside another group.

If you have a lot of materials you can find which material they are in with a script like this -

import bpy

for mat in bpy.data.materials:
    for mat_node in mat.node_tree.nodes:
        if mat_node.type == 'GROUP' and mat_node.node_tree.name == 'BadGroupName':
            print ("nodegroup in "+mat.name)


If your sure you don't want the group you could remove it instead of showing the material name, I'd rather look to make sure deleting it doesn't break things unexpectedly. You could also select the group node and bring it into view.